This invention relates in general to the field of drive methods for use with a thermal recording apparatus, such as a printer or facsimile machine, and in particular to application of a thermal printhead to the gray scale printing process in conjunction with a high speed interface.
Current facsimiles typically print only black and white images. A dither gray scale can be obtained by printing a mosaic of black and white dots which appear to the user as a gray level (due to the integrating properties of the human eye). However, true gray scale images provide a vast improvement over the dither gray scale in that the resolution is much higher, and the information associated with the image is not destroyed by the dithering processing.
A typical thermal printhead consist of a shift register (e.g., 1728 elements long) a latch (e.g., 1728 elements long), segment drivers (e.g., four) and image resistors (e.g., 1728 resistors grouped into four segments, three of which contain 448 resistors, and one which contains 384 resistors).
There are two categories of thermal printhead, thick film, and thin film. In thick film printheads, image resistors are screened onto the head, which makes for relatively large and uneven resistors whose resistance values vary across the printhead. Relatively large resistors take longer activation times to heat sufficiently for thermal printing. When a thick film printhead is used to print a gray scale image, dark and light streaks often appear across what should be an even gray image.
A thin film printhead uses photo-lithographic processes to deposit and etch resistive material on the printhead. The thin film method creates a printhead with relatively small resistors having less variation across the printhead elements. The smaller resistors of thin film printheads take less time to heat up compared with the resistors of thick film printheads. When thin film printheads are used to print gray scales no streaking is apparent.
In order to meet military specifications and other requirements (e.g., MIL-STD-188-161A, MIL-STD-188-161B, and NATOSTANAG-5000), a facsimile must be able to support 16 grade shades printing, and be able to print one line (1728 picture elements) in 10 milliseconds. This significant time constraint includes the time required to step the paper forward in order to print the next line.
Previous methods to provide for gray scale printing have been described involving amplitude modulating the printhead voltage, using a single stylus impact head to achieve 8 levels of gray, printing one gray level at a time followed by 16 outputs, or printing one bit at a time followed by 16 outputs (the latter two examples assuming 16 gray shades). These methods, which typically vary strobing timing do not meet the 10 millisecond per line time constraint.